Mono-substituted silicotungstates as active catalysts for sustainable oxidations: Homo- and heterogeneous performance

Article abstract of DOI:10.1039/c3nj00243h

A series of tetrabutylammonium (TBA) salts of the transition metal mono-substituted silicotungstates [SiW₁₁M(H₂O)O ₃₉]^n-, M = Co^II, Fe^III, Mn ^III, (SiW₁₁M) were explored as homogeneous catalysts for the oxidation of geraniol and styrene with H₂O₂. The most active homogeneous catalysts (SiW₁₁Co and SiW₁₁Fe) were immobilized onto an amine-functionalized SBA-15 (aptesSBA-15) and the resulting composites were characterized using several techniques (FT-IR, FT-Raman, UV-Vis/DRS, elemental analysis, powder XRD, SEM and N₂ adsorption-desorption isotherms). The catalytic performance of the new composites SiW₁₁Co@aptesSBA-15 and SiW₁₁Fe@aptesSBA-15 was investigated under similar experimental conditions to those used for homogeneous counterparts. 2,3-Epoxygeraniol and benzaldehyde were the main products obtained from geraniol and styrene oxidation, respectively, for all the catalysts. SiW₁₁Co and SiW₁₁Co@aptesSBA-15 showed to be the most active catalysts for the oxidation of geraniol and styrene. The recyclability of the composite SiW₁₁Co@aptesSBA-15 was investigated for three reaction cycles. The stability of the composites was confirmed using several techniques after catalytic cycles.

Full text of DOI:10.1039/c3nj00243h

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Mono-substituted silicotungstates as active catalysts
for sustainable oxidations: homo- and heterogeneous
performance†
Cite this: DOI: 10.1039/c3nj00243h
a
a
b
c
Salete S. Balula,* Lu ´ı s Cunha-Silva, Isabel C. M. S. Santos, Ana C. Estrada,
d
b
d
a
A. C. Fernandes, Jos e´ A. S. Cavaleiro, Jo a˜ o Pires, Cristina Freire and
b
Ana M. V. Cavaleiro
A series of tetrabutylammonium (TBA) salts of the transition metal mono-substituted silicotungstates
nÀ
II
III
III
[
SiW11M(H
2
O)O39
]
, M = Co , Fe , Mn , (SiW11M) were explored as homogeneous catalysts for the
. The most active homogeneous catalysts (SiW11Co and
oxidation of geraniol and styrene with H
2 2
O
SiW11Fe) were immobilized onto an amine-functionalized SBA-15 (aptesSBA-15) and the resulting
composites were characterized using several techniques (FT-IR, FT-Raman, UV-Vis/DRS, elemental analysis,
2
powder XRD, SEM and N adsorption–desorption isotherms). The catalytic performance of the new
composites SiW11Co@aptesSBA-15 and SiW11Fe@aptesSBA-15 was investigated under similar experimental
conditions to those used for homogeneous counterparts. 2,3-Epoxygeraniol and benzaldehyde were the
main products obtained from geraniol and styrene oxidation, respectively, for all the catalysts. SiW11Co and
SiW11Co@aptesSBA-15 showed to be the most active catalysts for the oxidation of geraniol and styrene.
The recyclability of the composite SiW11Co@aptesSBA-15 was investigated for three reaction cycles. The
stability of the composites was confirmed using several techniques after catalytic cycles.
Received (in NJBRA)
5
th March 2013,
Accepted 16th April 2013
DOI: 10.1039/c3nj00243h
www.rsc.org/njc
transition metal substituted POMs are attractive as catalysts for
oxidative reactions because they can be viewed as reactive low
1. Introduction
The search of new, efficient and selective catalytic systems valent transition metal centers complexed by inorganic oxometalate
capable of oxidising organic compounds under sustainable ligands, which have high capacity as oxygen transfer agents.1,3–6
conditions is a research area of high interest. Catalytic epoxidation
Besides the dioxygen, hydrogen peroxide is one of the most
of alkenes and monoterpenes has attracted much attention for attractive oxidants because it is environmentally clean and easily
7
organic syntheses, since their products are among the most useful handled, and the interest in these oxidants has been rising in
synthetic intermediates, as well as in industry processes. Catalytic polyoxometalate catalysis. Among the Keggin type silicotungstates,
function of polyoxometalates (POMs) has been a remarkable di-lacunary and di-metal-substituted anions have been reported as
research target due to their acidic and redox properties, which active catalysts for the oxidation of several hydrocarbons with
1
–3
8À
3,8–14
can be controlled at atomic or molecular levels. In particular, H O . In particular, the divacant [g-SiW O ] ion
and several
dimetal-substituted [g-SiW M O ] anions (M = Fe, Mn, Ti,
2
2
10 36
nÀ
2 36
1
0
a
15–21
REQUIMTE, Departamento de Qu ´ı mica e Bioqu ´ı mica, Faculdade de Ci ˆe ncias,
Universidade do Porto, 4169-007 Porto, Portugal. E-mail: sbalula@fc.up.pt;
Fax: +351 220402659; Tel: +351 220402576
Departamento de Qu ´ı mica, QOPNA, Universidade de Aveiro, 3810-193 Aveiro,
Portugal
Cu)
have been the most investigated. Less attention has
been paid to the catalytic performance of mono-substituted silico-
tungstates as oxidative catalysts. To the best of our knowledge, only
a few publications have addressed this family of POMs using
oxygen as oxidant. The first work was published in 2000 and
b
c
Departamento de Qu ´ı mica, CICECO, Universidade de Aveiro, 3810-193 Aveiro,
Portugal
6À
consists of the oxidation of p-xylene using [SiW M(H O)O ]
11
2
39
d
II
II 22
Departamento de Qu ´ı mica e Bioqu ´ı mica, CQB, Faculdade de Ci ˆe ncias,
Universidade de Lisboa, Campo Grande C8, 1749-016 Lisboa, Portugal
Electronic supplementary information (ESI) available: Additional crystal struc-
with M = Co and Cu . Later, Yamaguchi and Mizuno published
the oxidation of different alkanes and alcohols using the ruthe-
†
5
À
nium substituted anion [SiW11Ru(H
2
O)O39
]
as active catalyst with
2
ture figures; FT-IR, FT-Raman and diffuse reflectance spectra; N isotherms and
23
molecular oxygen. Kortz et al. described the aerobic oxidation of
SEM images. CSD 424106. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3nj00243h
5
À
24
cyclohexane by [SiW11Fe(H
2
O)O39
]
using microwave radiation.
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Raman (FT-Raman) spectra were recorded on a RFS-100 Bruker
FT-spectrometer, equipped with a Nd:YAG laser with excitation
wavelength of 1064 nm, and laser power set to 200 mW.
Diffuse reflectance spectra (UV-Vis/DRS) were registered on a
Jasco V-560 spectrophotometer, using MgO as reference.
Nitrogen (Air Liquide, 99.999%) physisorption experiments
were made at À196 1C using a volumetric apparatus (NOVA
2200e, surface area and pore size analyzer). For every experi-
ment, about 50 mg of the sample was degassed for 2.5 h at
150 1C at a pressure lower than 0.133 Pa.
Scanning electron microscopy (SEM) analysis was performed
at ‘‘CEMUP – Centro de Materiais da Universidade do Porto’’
Porto, Portugal), using a high resolution environmental scanning
Scheme 1
(
electron microscope (FEI Quanta 400 FEG ESEM). The samples
were analysed as powders.
Using H
2
O
2
as oxidant, we have performed a few studies for the
oxidation of different alkane substrates, using the mono-substituted
silicotungstates as active homogeneous catalysts.
indicated that monosubstituted silicotungstates were inactive in
catalytic epoxidations with H O in biphasic systems (under the
conditions of the Ishii–Venturello epoxidation system). The lack of
interest in the use of these silicotungstates in catalytic epoxidations
with H O may possibly stem from these previous results. The work
2 2
presented in this paper demonstrates that it is worthwhile to
perform catalytic studies using these catalysts.
The difficulty of active homogeneous catalyst recyclability
and the product separation are present concerns that can be
solved by the immobilization of the active species on solid
supports. Many supports have been referred to previously in
studies with POMs, but only a few published studies report the
immobilization of POMs on SBA-15
report the immobilization of silicotungstates on this support.
In the present work, we describe the oxidation of geraniol (i)
and styrene (iv) by H O in MeCN (Scheme 1), catalysed by various
25–28
Powder XRD analyses were made in a Philips Analytical PW
Some reports
3
050/60 X’Pert PRO (y/2y) diffractometer equipped with an
X’Celerator detector and with automatic data acquisition
X’Pert Data Collector v2.0b software) using a monochromatized
2
2
(
29
CuKa radiation (l = 1.5406 Å). Diffractograms were obtained by
continuous scanning from 0.331 to 5.01 2y, with a step size of
0.0331 2y, and 40 kV and 40 mA in the X-ray tube.
The GC-MS analyses were performed on Finnigan Trace GC-MS
(
Thermo Quest CE instruments) using helium as the carrier gas
À1
(
35 cm s ); GC-FID was performed using a Varian Star 3400CX
chromatograph to monitor homogeneous reactions, and a Varian
CP-3380 to follow the heterogeneous reactions. The hydrogen was
À1
used as the carrier gas (55 cm s ) for the first case and for the
second helium was utilized. In both cases, fused silica Supelco
capillary columns SPB-5 (30 m  0.25 mm i.d.; 25 mm film
thickness) were employed.
30–34
and only two publications
32,33
2
2
2.2. Synthesis of silicotungstates
mÀ
transition metal mono-substituted [SiW M(H O)O ] , M =
11
2
39
III
III
II
The potassium salts of mono-substituted silicotungstates were
prepared from the commercial silicotungstic acid at pH 6 in
the presence of the transition metal to be incorporated in the
Mn , Fe , Co (SiW M). Furthermore, the most active homo-
11
geneous silicotungstates were immobilized onto amine-modified
SBA-15 (aptesSBA-15) support material. The catalytic activity,
the recycle capability and the stability of SiW11M@aptesSBA-15
silicotungstate framework. The detailed procedures are well
III
described in the literature:
K
5
[SiW11Fe (H
2
O)O39]Á10H
2
O
(M = Co and Fe) were investigated.
III
(
KSiW11Fe),
[SiW11Co (H
butylammonium (TBA) salts were obtained from the respec-
K
5
II
[SiW11Mn (H
2
O)O39]Á7H
2
O (KSiW11Mn) and
35–38
K
4
H
2
2
O)O39]Á22H
2
O (KSiW11Co).
The tetra-
2. Experimental
3
9,40
tive potassium salts using a procedure reported previously.
An aqueous solution of the previously prepared potassium
2.1. Materials and methods
Geraniol, styrene and (3-aminopropyl)triethoxysilane (aptes) silicotungstate and a solution of TBABr in 1,2-dichloroethane
were purchased from Aldrich and used without further purifi- were mixed and vigorously stirred. The organic phase was
cation. Acetonitrile (99.5% Panreac), 30% H
other reagents were used as received.
2
O
2
(Aldrich) and collected and the 1,2-dichloroethane removed by evaporation.
The so-obtained oil was then dissolved in acetonitrile and the
Metal analysis for K, W, Fe, Mn, Co was performed using ICP resultant tetrabutylammonium compound was precipitated
spectrometry (University of Aveiro) and C, H, N elemental by the addition of the minimum amount of water. All solids
analysis was performed on a Leco CHNS-932. Hydration water were characterised using elemental and thermal analysis,
contents were determined using thermogravimetric analysis on FT-IR and FT-Raman and electronic absorption spectroscopy
a TGA-50 Shimadzu thermobalance.
Fourier transform infrared (FT-IR) absorption spectra were those previously published:
4 2
obtained on a Mattson 7000 FT-IR spectrometer, using KBr (TBASiW11Fe), TBA H[SiW11Mn (H O)O39] (TBASiW11Mn), and
(solution and solid). The results were in agreement with
3
5–38,40
III
TBA H[SiW Fe (H O)O ]
4 11 2 39
III
À1
II
pellets. Spectra were collected in the range 400–4000 cm
,
TBA
4
H
2
[SiW11Co (H
2
O)O39]ÁH
2
O (TBASiW11Co). The only excep-
[SiW11CoH O,
40]Á22H
À1
using a resolution of 4 cm and 32 scans. Fourier transform tion was the KSiW11Co compound, K
H
4 2
2
O
2
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+
that was obtained with a different molecular formula (four K
under argon. The obtained solid was characterized using C,H,N
were found instead of six). Anal (wt%). Found: K, 5.21; W, 61.0; elemental analysis, FT-IR, FT-Raman spectroscopy, powder XRD,
Co, 1.70; H O, 12.8% calcd: K, 4.73; W, 61.19; Co, 1.78; H O, analysis of N2 adsorption isotherms and SEM analysis. The
2
2
À1
1
2.1%. FT-IR (cm ) 999 (m), 965 (vs), 914 (vs), 815 (vs, sh), resulting aptesSBA-15 contained 1.2 mmol of NH per 1 gram of
2
7
19 (s), 540 (vs).
material. The immobilization of silicotungstates was performed
by stirring the mixture of 0.5 g of amine-modified SBA-15 in 50 mL
of water with 0.4 mmol of KSiW11Co and KSiW11Fe, for 24 h at
2.3. Single crystal X-ray analysis
Crystalline material of KSiW11Co was harvested from the crystal- room temperature. The resulting materials were filtered and
lization vial, immediately immersed in viscous oil, and a suitable washed with water until the filtrate did not show any POM
4
1
single-crystal was selected and mounted on the CryoLoop. Data (monitored using UV-Vis spectroscopy). Finally, the solids were
were collected on a Bruker X8 Kappa APEX II charge-coupled dried under vacuum for several hours at 100 1C. The obtained
device (CCD) area-detector diffractometer (Mo Ka graphite- composites were characterized using the techniques mentioned
monochromated radiation, l = 0.71073 Å) controlled by the above as well as diffuse reflectance spectroscopy and ICP analysis.
4
2
APEX2 software package, and equipped with an Oxford Cryo- The loading of silicotungstates achieved was 110 mmol (22 wt% of
systems Series 700 cryostream controlled with a Cryopad inter- W) of SiW11Co for SiW11Co@aptesSBA-15 and 89 mmol (18 wt% of
4
3
44
face. Images were processed using SAINT+, and data were W) of SiW11Fe for SiW11Fe@aptesSBA-15, per gram of material.
corrected for absorption using the multi-scan semi-empirical
4
5
2.5. Oxidation reactions
method implemented in SADABS. The structure was solved
46,47
using the direct methods implemented in SHELXS-97,
allowing The oxidation reactions of geraniol (i) and styrene (iv)
the immediate location of most of the heaviest atoms, and all the (Scheme 1) were carried out in MeCN, in a borosilicate 10 mL
remaining non-H atoms were located from difference Fourier maps reaction vessel, with addition of aqueous H (30 wt%), in the
calculated from successive full-matrix least squares refinement presence of TBA salts of the silicotungstate (SiW11M, M = Fe ,
2 2
O
III
2
46,48
III
II
cycles on F using SHELXL-97.
All non H-atoms were success- Mn , Co ) or of the silicotungstate composites (27 mg of
+
fully refined using anisotropic displacement parameters. Four K
SiW11Co@aptesSBA-15 and 34 mg of SiW11Fe@aptesSBA-15).
cations for each silicotungstate anion were directly located from In the oxidation of (i) the reaction was performed at room
difference Fourier maps, thus the charge of the silicotungstate is temperature and protected from light. The oxidative reactions
then supposedly compensated by the presence of H-atoms bound of (iv) were carried out at 80 1C. In a typical experiment, MeCN
to the anion. Nevertheless, these charge-balancing H-atoms and (1.5 mL) was added to the substrate (1 mmol) and the catalyst
those belonging to the water molecules of crystallization could not (3 mmol) was placed in the reaction vessel and stirred. H O was
2 2
be located from difference Fourier maps or even placed in calcu- added to the reaction mixture, 500 ml (4.5 mmol) for the
lated positions. However, all these H-atoms were added to the oxidation of (i) and (iv). The reactions were followed by GC
empirical formula.
analysis and stopped when a complete conversion of the
O, substrate was observed or when the product yields remained
2
Crystal data for KSiW11Co: K
4
H
2
[SiW11CoH
2
O
40]Á22H
M = 3306.15, tetragonal, space group P4/mnc, a = 14.1182(7) Å, constant after two successive GC analysis. At regular intervals,
3
b = 14.1182(7) Å, c = 12.4604(11) Å, a = b = g = 90.001, V = 2483.7(3) Å , an aliquot was taken directly from the reaction mixture using a
À1
T = 150(2) K, Z = 2, m = 26.184 mm , 11494 reflections measured, microsyringe, diluted in MeCN, centrifuged (if necessary) and
1
333 independent reflections (Rint = 0.0486). The final R values were injected into the GC or GC-MS equipment for analysis of
1
2
0
.0382 [I 4 2s(I)]. The final wR(F ) values were 0.0952 [I 4 2s(I)]. starting materials and products. The reaction products
2
51,52
The final R
1
values were 0.0409 (all data). The final wR(F ) values reported here were identified as described elsewhere.
The
were 0.0971 (all data). Crystallographic information (excluding heterogeneous catalyst was filtered off at the end of reactions,
structure factors) can be obtained free of charge via http://www. washed with MeCN several times to remove the remaining
fiz-karlsruhe.de/obtaining_crystal_structure_data.html or from substrate, reaction products and oxidant. The recovered
the Inorganic Crystal Structure Database (ICSD, FIZ Kalsruhe, catalyst was dried at room temperature overnight and reused
Hermannvon- Helmholtz-Platz 1, Eggenstein-Leopoldshafen, 76344, in a new reaction under identical experimental conditions, with
Germany; phone: +49 7247808555, fax: +49 7247808259; e-mail: readjustment of all quantities, without changing the molar
crysdata@fiz-karlsruhe.de), on quoting the depository numbers ratios and reaction concentrations. Blank reactions were performed
CSD-424106.
for all substrates, confirming that no oxidation products are
obtained unless the catalyst and H O are present. Even in the
presence of the support aptesSBA-15 no oxidation products were
2
2
2.4. Immobilization of silicotungstates onto SBA-15
SBA-15 was obtained following a procedure adapted from the obtained.
literature, using the block copolymer Pluronic P123 and tetraethyl
orthosilicate (TEOS) under acidic conditions. The template was
3
. Results and discussion
4
9
removed by calcination at 550 1C. The surface of SBA-15 was
3.1. Catalysts preparation and characterization
modified by post-grafting methodology with (3-aminopropyl)-
triethoxysilane (aptes), using a procedure reported previously.
50
The potassium and the TBA compounds of the anions
The dried support was refluxed for 24 h in dry toluene with aptes [SiW11M(H
nÀ
III
III
II
2
O)O39
]
(M = Mn , Fe , Co ) have been described
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3
5–38,40,53
previously.
2H O (KSiW11Co) was obtained in this work, instead of the more
common K [SiW Co(H O)O ]Á13H O usually referred in the litera-
Still, the compound K
4
H
2
[SiW11Co(H
2
O)O39]Á
2
2
6
11
2
39
2
3
7
ture. Furthermore, crystalline material of the KSiW Co suitable
11
for single-crystal X-ray diffraction (XRD) analysis could be grown.
The crystal structure was determined in the tetragonal P4/nmc space
group (for details concerning the crystallographic data collection
and refinement see Experimental section), being isostructural with
54
related compounds such as K
and K [SiW Mo Co(H
O)O39]Á22(H
parison of the structure of K H [SiW Co(H O)O ]Á22H O with that
4
H
2
[SiW
4
Mo
7
Ni(H
2
O)O39]Á13(H
2
O)
5
5
4
H
2
7
4
2
2
O). Furthermore, the com-
4
2
11
2
39
2
of K [SiW Co(H O)O ]Á13H O based on the powder XRD patterns
6
11
2
39
2
5
6,57
+
reveals substantial similarities.
cations, Keggin type [SiW11Co(H
The structure is build up of K
6À
2
O)O39
]
anions (Fig. S1 in the
ESI†) and hydration water molecules (Fig. S2, ESI†). As a conse-
quence of the 4/m symmetry, each oxygen atom of the central SiO
Fig. 1 FT-IR spectra of support aptesSBA-15 and silicotungstate composites in
À1
4
the wavenumber region between 400 and 1900 cm . Name of samples with ac
group of the polyoxoanion is disordered over two half-occupied corresponds to recovered samples after catalyzing the styrene oxidation reaction.
sites, corresponding to a 901 relative rotation of the Keggin anions.
Furthermore, as a consequence of the high symmetry of this
6
1
amine groups on the aptesSBA-15 material. Comparing the
spectrum of the support aptesSBA-15 with the spectra of
SiW Fe@aptesSBA-15 and SiW Co@aptesSBA-15 it is possible
structure and the molecular disorder it is not possible to differen-
II
VI
tiate the Co centre from the eleven W centres, thus the Co and
eleven W atoms were equally distributed and refined in the twelve
metallic positions of the polyoxoanion (Fig. S1, ESI†).
1
1
11
to find additional bands that indicate the presence of the
immobilized silicotungstate on the support. One of the charac-
teristic bands that can be identified in the spectra of composite
SBA-15 was chosen as support material to prepare the
heterogeneous catalysts due to its large surface area, easy
surface modification with functional groups, high stability
and low toxicity. This support was functionalized by reaction
À1
materials appears at ca. 920 cm and is attributed to nas(W–O–W)
and nas(Si–O) that cannot be distinguished by their wavelength
proximity (Fig. 1). The infrared spectra of isolated silicotungstates
SiW11Co and SiW11Fe are presented in Fig. S3, ESI.†
5
0
with the organosilane aptes in a post-synthesis step, and the
silicotungstate SiW Fe and SiW Co were immobilized on the
1
1
11
The FT-Raman spectra of the immobilized silicotungstates are
more elucidative than the FT-IR data because the silica support
does not show any band on the region of the Keggin silicotung-
states. Similar patterns are found for SiW11Fe@aptesSBA-15 and
SiW11Co@aptesSBA-15 when compared with those of the TBA
À1 salts of SiW Fe and SiW Co and their characteristic bands are
amine-functionalized SBA-15 (aptesSBA-15) through a procedure
previously used to prepare supported phosphotungstates. The
amount of amine groups on the surface of the material was
3
4
determined by elemental analysis of C and N (1.2 mmol of NH
2
per 1 gram of material) and the amount of silicotungstate by the
À1
11
11
analysis of tungsten (0.11 mmol g of SiW Co, 0.089 mmol g
1
1
seen without any interference due to the support (Fig. 2 and Fig. S5,
ESI†). The spectra of SiW11Fe and SiW11Co show a strong band
of SiW Fe). Furthermore, the metal analysis of W, Co and Fe
1
1
suggests that no degradation of the structure of the transition
metal substituted silicotungstate occurred since the molar ratio
of W/Co or W/Fe is approximately 11 in both cases. The
immobilization of mono-substituted silicotungstates SiW11Fe
and SiW11Co can occur by the coordination of the transition
III
II
metal (Fe and Co ) with the amine group present on the
support material, as suggested before in the literature for other
mono-substituted polyoxometalates immobilized onto various
3
1,58,59
amine-functionalized silica supports.
Other hypothesis is
6
0
the electrostatic interaction with protonated amine groups.
The presence of immobilized silicotungstates and the stability
of the aptesSBA-15 material after immobilization were established
using various spectroscopic methods, including UV-Vis/DRS, FT-IR
and FT-Raman. Fig. 1 and Fig. S4 in ESI† present the FT-IR spectra
of the immobilized silicotungstates and the support aptesSBA-15.
À1
The stronger bands in the region of 400–1100 cm are attributed
to the SBA-15 n (Si–O–Si), n (Si–O–Si) and d(O–Si–O) which overlap
as
s
the characteristic bands of the silicotungstates (Fig. S3, ESI†).
In the region of 1500–1580 cm two weak bands assigned to the
Fig. 2 FT-Raman spectra of aptesSBA-15 and silicotungstate composites in the
À1
À1
wavenumber region between 300 and 1500 cm . Name of samples with ac
NH
2
groups can be observed, which confirms the incorporation of corresponds to recovered samples after catalyzing the styrene oxidation reaction.
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Fig. 3 Diffuse reflectance electronic spectra: (a) TBASiW11Co, (b) SiW11Co@
aptesSBA-15 and (c) SiW11Co@aptesSBA-15-ac, where ac means after catalyzing
the styrene oxidation reaction.
Fig. 4 Powder X-ray diffraction patterns for the indicated samples.
À1
near 1000–970 cm attributed to n (W–O) that corresponds to the
s
39,62
bond between tungsten and the terminal oxygen.
This band is
slightly shifted to lower wavenumbers when silicotungstates are
supported onto aptesSBA-15 (Fig. S5, ESI†).
The UV-Vis/DRS spectra of SiW11Co@aptesSBA-15 and
SiW11Fe@aptesSBA-15 reveal a band with lmax ca. 270 nm,
which is attributed to oxygen-to-tungsten charge transfer from
6
3
W–O–W bonds of silicotungstates (Fig. 3 and Fig. S6, ESI†).
Furthermore, the spectra of SiW Co and the composite
1
1
SiW11Co@aptesSBA-15 can be compared to evaluate possible
II
alterations of Co coordination environment after immobilization.
The spectrum of SiW11Co shows one splitted band with lmax ca.
3
7
5
40 nm that corresponds to d–d electronic transition. However,
the same band is enlarged, with lmax shifted to higher wavelength
ca. 570 nm) when the same compound was immobilized on
aptesSBA-15, which may indicate an alteration of the coordina-
(
Fig. 5 Nitrogen adsorption–desorption isotherms at À196 1C for the initial and
modified SBA-15 samples. Name of samples with ac corresponds to recovered
samples after catalyzing the styrene oxidation reaction.
II
II
tion sphere of Co (Fig. 3). A shift of the d–d band from Co
incorporated into phosphotungstate structures was observed
before when the compound was immobilized on amino-modified
31,58
mesoporous silica.
SiW11Fe@aptesSBA-15 (Fig. S6, ESI†) show charge transfer bands
O - W and O - Fe).
The UV-Vis/DRS spectra of SiW11Fe and
with aptes and so the specific surface area is lowered to near 50%
of the initial value (Table 1). Nevertheless, after grafting the
silicotungstates in the aptesSBA-15 sample, only a small reduction
in the surface area and porous volume is noticed, meaning that a
considerable amount of porous volume (near 0.5 cm g ) is still
available in the prepared catalysts.
The pore size distributions (Fig. S7, ESI†) were obtained
using the DFT method and reveal a shift for lowest pore sizes
for the materials upon functionalization/grafting and also some
(
The results obtained from the analysis of FT-IR, FT-Raman
and UV-Vis/DRS comparing the isolated silicotungstates
and the supported silicotungstates suggest that SiW Fe and
SiW11Co are effectively immobilized on the aptesSBA-15 and no
degradation of silicotungstates structure occurred.
3
À1
1
1
Powder XRD patterns are depicted in Fig. 4. The strong
6
4
reflection characteristic of SBA-15 type materials is displayed
and, as overall, the peaks can be indexed on a hexagonal lattice
as reflections (100) (110) and (210). The results confirm the
integrity of the SBA-15 after the various steps of functional-
ization and supporting the POMs.
Table 1 Textural properties of the solid support and heterogeneous catalysts
Total pore
volume (cm g
2
À1
3
À1
A
BET (m g
)
)
The nitrogen adsorption–desorption isotherms at À196 1C
65
SBA-15
aptesSBA-15
SiW11Co@aptesSBA-15
763
380
298
62
0.976
0.650
0.497
0.122
present the usual shape for SBA-15 type of materials, and can
66
be classified as Type IV with H1 hysteresis loop (Fig. 5). The
adsorbed amounts are reduced upon the surface functionalization SiW11Co@aptesSBA-15 ac
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broadening of the distributions. In general, the results for of geraniol oxidized and the amount of H
2
O
2
consumed during
the pore size distributions are compatible with the presence 6 h of reaction. The efficiency of H
2 2
O usage was observed to
of the silicotungstates mainly inside the porous structure of the decrease with reaction time and 94, 80 and 52% were obtained
SBA-15 material.
after 30 min, 1 h and 6 h, respectively.
Few studies have been published for the oxidation of geraniol
catalysed by Keggin type POMs and most of them have been
3.2. Homogeneous catalysis
67–69
performed using homogeneous systems.
Recently, some
The oxidation of geraniol and styrene was carried out in metal mono-substituted phosphotungstates and borotungstates
homogeneous phase using H as oxidant and MeCN as were used to oxidise selectively geraniol to 2,3-epoxygeraniol
solvent, in the presence of catalytic amounts of TBA salts of under similar experimental conditions used in this work.
2 2
O
3
4,67,68
III
III
II
silicotungstates SiW11M (M = Mn , Fe and Co ). All the However, in the presence of SiW Co higher substrate conver-
1
1
silicotungstates could catalyse the reactions studied (Fig. 6). sion was achieved in this work for less reaction time.
In the absence of a catalyst the conversion never exceeded 7%
The oxidation of styrene catalysed by silicotungstates yielded
after 24 h of reaction.
benzaldehyde (v) as the main product. Benzaldehyde is the only
The main product obtained from the oxidation of geraniol (i) product detected during the first 30 min of reaction in the
with H O at room temperature was 2,3-epoxygeraniol (ii, presence of all catalysts; however, its selectivity decreases with
2
2
Scheme 1) in the presence of all catalysts, with selectivity always the increase of the reaction time because other products such
higher than 90%, indicating the highly chemoselective nature as acetophenone (vi), phenylacetaldehyde (viii) and benzoic
of the reactions. A small amount of diepoxide (iii) was found acid (vii) are also produced (Scheme 1). Table 3 shows the
after 6 h of reaction in the presence of all studied catalysts product distribution and the catalytic performance obtained for
(Table 2). The best catalytic performance was observed for styrene oxidation after 6 and 24 h in the presence of different
SiW Co which attained 95% of conversion and 100% of silicotungstates. The best catalytic performance was found for
1
1
selectivity after 30 minutes of reaction. After 6 h of reaction, SiW Co where after 6 h a complete conversion was obtained.
1
1
almost complete conversion of geraniol was achieved in the This catalyst showed a fast activity since the first minute of
presence of all catalysts. The efficiency of the usage of H was reaction and after 30 min a conversion of 62% was achieved
2 2
O
analyzed for the oxidation of this substrate catalysed by with 80% of selectivity to benzaldehyde. At this stage phenyl-
SiW11Co. Fig. S8 in ESI† display the comparison of the amount acetaldehyde, acetophenone and benzoic acid are also produced.
The SiW Co has been studied before as a homogeneous catalyst
1
1
for styrene oxidation under different experimental conditions;
however, better catalytic results were found in our work when we
increased the ratio substrate/oxidant to 1 : 4.5 and when the
3
3
volume of MeCN was decreased to 1.5 mL. Using SiW11Fe as
catalyst a complete conversion of styrene was achieved; however,
7
0
2 2
larger H O excess and long reaction time were required.
The oxidation of styrene to benzaldehyde with H O cata-
2
2
lyzed by polyoxotungstates and peroxophosphotungstates is
3
3,71–73
already well documented in the literature.
The mechanism
2 2
O with the POM,
proceeds initially by the interaction of H
generating active species which may be hydroperoxo or bridging
Fig. 6 Conversion data obtained for the oxidation of different substrates after
h of reaction when catalysed by homogeneous silicotungstates: (a) SiW11Co, (b)
1
Table 3 Data for the oxidation reaction of styrene in the presence of different
SiW11Fe, (c) SiW11Mn; and after 24 h when catalysed by the composites (d)
SiW11Co@aptesSBA-15 and (e) SiW11Fe@aptesSBA-15. Reaction conditions:
ab
catalysts
3
8
mmol of silicotungstate, 1 mmol substrate, 4.5 mmol H
2 2
O , 1.5 mL MeCN,
c
Select. (%)
0 1C (room temperature and light protection in the presence of geraniol).
Catalyst
Conv. (%)
v
vi
vii
viii
No catalyst
SiW11Co
SiW11Fe
SiW11Mn
SiW11Co@ aptesSBA-15
SiW11Fe@ aptesSBA-15
E0
99
—
75
—
7
—
7
—
11
7 (9)
Table 2 Conversion (%) and selectivity (%) for the oxidation reaction of geraniol
a
with H
2
O
2
using homogeneous silicotungstate catalysts
68 (100)
19 (22)
58 (100) 100 (75)
41 (98) 100 (52)
74 (65) 10 (19) 9 (7)
58 (55)
b
b
(27)
(25)
(48)
(9) 42 (9)
—
—
Catalyst
Conv. 1 h (select.)
Conv. 6 h (select.)
—
—
SiW11Co
SiW11Fe
SiW11Mn
97 (95)
46 (94)
54 (98)
97 (94)
99 (96)
99 (95)
a
Conversion and selectivity data at 6 h of reaction, except values in
b
parentheses, obtained after 24 h of reaction. Reaction conditions:
a
2 2
Reaction conditions: 1 mmol of geraniol, 4.5 mmol H O
2 2
, 3 mmol of 1 mmol of styrene, 4.5 mmol H O , 3 mmol of silicotungstate or the
silicotungstate, 1.5 mL MeCN, room temperature and light protection. amount of composite containing 3 mmol of silicotungstate, 1.5 mL
b
c
Based on the amount of consumed substrates; corresponding to MeCN and 80 1C. Selectivity for the various products obtained from
2
,3-epoxygeraniol.
styrene oxidation assigned in Scheme 1.
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peroxo species. Afterwards, styrene is bonded with one of the
metal–peroxo bonds to produce a peroxometallocycle and in the
next step styrene oxide is formed. A further nucleophilic attack of
33,72
H O on the styrene oxide originates benzaldehyde.
The
2
2
phenylacetaldehyde is possibly formed through isomerisation of
styrene oxide, while the formation of benzoic acid from benzal-
74
dehyde is a simple oxidation.
3.3. Heterogeneous catalysis and recyclability
The catalytic performance of SiW Co@aptesSBA-15 and
1
1
Fig. 8 Activity data obtained for the oxidation of styrene after 24 h of reaction
when catalysed by the homogeneous SiW11Co and by the heterogeneous
SiW11Co@aptesSBA-15 for three consecutive cycles.
SiW Fe@aptesSBA-15 was evaluated for the oxidation of geraniol
11
and styrene under the same experimental conditions used for the
homogeneous catalytic systems (oxidant 30% H and MeCN as
2 2
O
solvent). Also for the heterogeneous catalytic systems the cobalt
based catalyst (SiW11Co@aptesSBA-15) was more active than the
SiW11Fe@aptesSBA-15 (Fig. 6). Comparing the performance of the
heterogeneous catalyst with the corresponding homogeneous
catalyst it is possible to notice that the first showed to be less
active for the oxidation of both substrates. From the oxidation of
geraniol (i) only the 2,3-epoxygeraniol (ii) was produced. The
benzaldehyde (v) was the main product obtained from styrene
oxidation and only acetophenone (vi) was formed as a by-product
room temperature to be used in a fresh reaction under identical
experimental conditions. The activity of SiW11Co@aptesSBA-15
and SiW11Fe@aptesSBA-15 is retained from the first to the
second reaction cycle; however, a decrease of activity was found
in the third reaction cycle. Fig. 8 shows the reusability of the
heterogeneous system SiW11Co@aptesSBA-15 for styrene oxidation
after 24 h of reaction. The selectivity for benzaldehyde is practically
maintained from the first to the second reaction cycle, having only
acetophenone as a by-product (Fig. 8). For the third cycle the
selectivity to benzaldehyde is decreased because benzoic acid (26%
of yield of (vii) in Scheme 1) is also formed.
(Table 3). During the first six hours of styrene oxidation reaction a
decrease of activity was observed when the heterogeneous systems
were used instead of the homogeneous compounds (Fig. 7).
However, after 24 h a complete conversion was attained by the
homogeneous and heterogeneous catalytic systems (Fig. 7).
Many cobalt(II)-catalyzed systems for oxidation of alkenes
with molecular oxygen (often involving a co-reductant, such as
For geraniol oxidation no conversion was found during the
first 2 h of reaction for the heterogeneous systems. However,
the difference of activity between homogeneous and hetero-
geneous systems became smaller with the reaction time, mainly
for SiW11Co@aptesSBA-15 that achieved near complete conver-
sion after 24 h (Fig. 6). The induction period observed in the
presence of the heterogeneous systems for geraniol oxidation
may be due to the less accessibility of the catalyst active centre.
The epoxidation of geraniol at the C –C position may involve
7
5–77
an aldehyde) have been reported.
The oxidation of styrene
yields usually styrene epoxide and benzaldehyde as main products,
with the outcome of the reactions varying with the conditions. The
oxidation of styrene by H O , catalyzed by cobalt complexes (in
2
2
homogeneous or heterogeneous conditions), is described in several
papers (see, for example, ref. 78–80). With these systems, benzal-
dehyde is usually the major product, as found in our work.
2
3
the formation of an intermediate complex between the metal
centre of the catalyst, the oxidant and the substrate, that should
be more difficult in the heterogeneous catalyst where the metal
78
Reported yields vary between 40 and 89%, showing that our
results are quite good.
8
1
centre is less accessible.
The solid catalysts can be easily separated by simple filtration
followed by washing (with MeCN and n-hexane) and drying at
To further investigate the catalytic activity of the hetero-
geneous catalyst versus the possible homogeneous reaction
with the leached active species from the support, the solid
catalyst was separated from the reaction mixture by careful
filtration after a certain time and the reaction was continued
further with the remaining filtrate. This experiment was performed
using the most active catalysts, i.e. SiW11Co@aptesSBA-15 for the
oxidation of styrene, where the catalyst was filtering after 1 h of
reaction. Fig. 9 shows the comparison of the kinetic profile of this
reaction with the case where the solid catalyst was not filtered. It
can be observed a slightly increase of conversion after solid catalyst
filtration what may probably indicate partial leaching of SiW Co
11
into the filtrate albeit it was small. The W analysis of the recovered
composite after catalytic use shows that 19% of the initial SiW11Co
loading in SiW11Co@aptesSBA-15 was leached after three conse-
cutive reaction cycles. According to this result, it will be important
to develop in the near future new strategies to avoid the leaching of
Fig. 7 Comparison of kinetic profiles for styrene oxidation of homogeneous
(
open symbols) and heterogeneous (solid symbols) reactions, using H
2 2
O as
oxidant and MeCN as solvent.
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4. Conclusions
The silicotungstates SiW11M revealed to be active catalysts for the
II
III
oxidation of geraniol and styrene in homogeneous (M = Co , Fe ,
III
II
III
2 2
Mn ) and heterogeneous (M = Co , Fe ) systems with H O as
an oxidant and MeCN as solvent. Compounds with the best
homogeneous catalytic performance were immobilized onto
amine-functionalized SBA-15 to prepare the heterogeneous cata-
lysts: SiW Co@aptesSBA-15 and SiW Fe@aptesSBA-15. Compar-
11
11
able activities between homogeneous and heterogeneous systems
were achieved after 24 h. The selectivity was higher using the
heterogeneous catalysts than the homogeneous catalysts. The
2 2
Fig. 9 Kinetic profiles for the oxidation of styrene with H O catalysed by
SiW11Co@aptesSBA-15 (1st run) and when the same was removed after 1 h of SiW11Co and the composite SiW11Co@aptesSBA-15 were the
reaction (leaching test).
most active catalysts for geraniol and styrene oxidation. 2,3-
Epoxygeraniol was the main product obtained from geraniol
oxidation (95% and 74% of conversion were obtained using
SiW Co after 30 min and SiW Co@aptesSBA-15 after 24 h,
respectively). Benzaldehyde was the main product obtained from
styrene oxidation and its selectivity was higher when the reaction
was catalyzed by the heterogeneous systems (58% of conversion
and 100% of selectivity for benzaldehyde were obtained after 6 h
using SiW11Co@aptesSBA-15 as catalyst).
The stability of the silicotungstate immobilized on aptesSBA-15
was verified after catalytic reactions and the integrity and recycle
capability of the support were investigated. The heterogeneous
character of the composites was evaluated by performing the
leaching analysis and a small loss of active centers seemed to
occur. When the silicotungstate composites were used in conse-
cutive reaction cycles for styrene oxidation a moderate decrease of
activity was observed from the second to the third cycle, which may
be caused by the loss of an active center found after the three
reaction cycles. The optimization of the catalytic systems will be
carried out to minimize the leaching active species and other
methodologies of SBA-15 functionalization will be investigated.
active species in order to immobilize more efficiently the silico-
tungstates on the SBA-15 support.
1
1
11
3.4. Stability analysis
To investigate the stability of catalysts after reaction cycles, the
materials SiW11Fe@aptesSBA-15 and SiW11Co@aptesSBA-15
were recovered after reaction of styrene oxidation and charac-
terized using different techniques. The FT-IR and the FT-Raman
spectra after catalytic runs reveal the same characteristic bands
of silicotungstates, corresponding to nas(W–O–W), nas(Si–O) and
ns(W–O), and also the bands of the support aptesSBA-15 (Fig. 1
and 2 and Fig. S4 and S5, ESI†). The UV-Vis/DRS spectra of
SiW Fe@aptesSBA-15 show the same charge transfer bands
1
1
(O - W, O - Fe) and the spectrum of SiW11Co@aptesSBA-15
II
exhibits the same d–d band of Co as observed before the
catalytic runs (Fig. 3 and Fig. S6, ESI†). The X-ray diffractograms
of SiW11Co@aptesSBA-15 plotted after catalytic runs display the
same peaks as before catalytic use, what confirm the integrity of
the SBA-15 after the various catalytic reaction cycles (Fig. 4). The
SiW Co@aptesSBA-15 composite was analysed using electron
1
1
Acknowledgements
microscopy (SEM). As observed in Fig. S8, ESI† the morphology
of the materials before and after catalytic use is similar confirming
the preservation of the aptesSBA-15 structure.
The authors acknowledge the Fundaç a˜ o para a Ci ˆe ncia e a
Tecnologia (FCT, MEC, Portugal) for general funding (PEst-C/
EQB/LA0006/2011 for Associated Laboratory REQUIMTE and
PEst-C/QUI/UI0062/2011 for QOPNA) and Pest-OE/QUI/UI0612/
The nitrogen adsorption–desorption isotherms at À196 1C
were also performed for the composites after catalysis (Fig. 5)
and it was possible to observe a reduction in the total porous
volume of SiW Co@aptesSBA-15 after catalysis. This reduction
2011 (CQB) and grant SFRH/BD/72058/2010(ACF).
1
1
in the specific surface area of the sample after catalysis seems
to be high but the significance of this value needs to be inter-
preted with care. In fact, it should be emphasized that the sample
was analyzed after being re-used in three catalytic cycles. At the
end of the catalytic cycles it is possible that solvent molecules, as
well as some reactant and product molecules, are occluded in the
pore structure. In the phase of cleaning under vacuum and
temperature, prior to the determination of the nitrogen adsorp-
tion isotherms, only a moderate temperature (150 1C) could be
used to avoid decomposition of the organic aptes surface groups.
These experimental conditions may not allow the complete
removal of the above mentioned molecules and, therefore, return
a low specific surface area value.
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